1. Field of the Invention
Aspects of the present invention relate to a lithium secondary battery, and more particularly, to a pouch type lithium secondary battery which includes a positive temperature coefficient element arranged therein to rapidly operate according to a temperature increase of the battery, thereby having improved thermal stability.
2. Description of the Related Art
Recently, compact and lightweight electric/electronic devices such as cellular phones, laptop computers, camcorders and the like have been actively developed and produced. These portable electric/electronic devices respectively contain a battery pack in order to operate in a place where a separate electric source is not supplied. The contained battery pack is provided with at least one battery therein, so as to output a certain level of voltage to drive the portable electric/electronic device for a given duration.
In a consideration of economics, the battery pack has recently employed rechargeable secondary batteries. Representative secondary batteries include a nickel-cadmium battery, a nickel-hydrogen battery, a nickel-metal hydride battery, a lithium battery, a lithium ion battery, etc.
Specially, the lithium secondary batteries have rapidly developed because their energy density per unit weight is high, and their operation voltage is 3.6V, which is three times higher than that of the nickel-cadmium battery and the nickel-metal hydride battery that are widely used as an electric source for the portable electronic devices.
In such a lithium secondary battery, a positive electrode active material is mainly made of lithium-based oxide, while a negative electrode active material is made of carbon material. Generally, the lithium secondary battery can be classified into a liquid electrolyte battery and a polymer electrolyte battery according to the kind of electrolyte. Accordingly, the battery using the liquid electrolyte is referred to as a lithium ion battery, while the battery using the polymer electrolyte is referred to as a lithium polymer battery. Further, the lithium secondary battery is manufactured into various types, for example, cylindrical type, polygonal type and pouch type.
Typically, the pouch type lithium secondary battery among the lithium secondary batteries has a pouch of multi-layers including a metal foil layer and a synthetic resin layer covering the metal foil layer. Thus, the pouch type lithium secondary battery employing such a pouch has a remarkably reduced weight in comparison with a cylindrical or polygonal lithium secondary battery using a metal can instead of the pouch. Thus, research for making lighter lithium secondary batteries has focused on the development of the pouch type lithium secondary battery.
In such a pouch type lithium secondary battery, an electrode assembly, in which positive and negative electrode plates and a separator interposed between the positive and negative electrode plates are wound in several times, is received in the pouch, while positive and negative electrode tabs are extended out of a side of the pouch to connect to a protection circuit module.
The protection circuit module has various protection circuits arranged on a printed circuit board. Further, the protection circuit module has a structure in which a positive temperature coefficient element is mounted thereon.
However, since the positive temperature coefficient element is mounted on the protection circuit module, the reaction velocity of the positive temperature coefficient element is slow when the temperature in the pouch type lithium secondary battery rapidly rises due to overcharging or rapid discharging of the battery.
If the reaction velocity of the positive temperature coefficient element is slow, as described above, there exists a danger of an explosion of the pouch type lithium secondary battery.